A drug is any substance that is used for cure, diagnosis, and prevention of diseases in man and other animals. For practical purposes, drugs and medicine are synonymous terms. Most people (practitioners and laymen alike) use the two terms without any marked distinctions.
Drugs have multiple effects on the body. These effects vary from person to person or for the same person on different occasions. It’s always worth reading the patient information leaflet that comes with your medicine and discussing any dietary concerns with your doctor or pharmacist.
Some of the reasons why the same medication can affect us differently include:
Body weights of individuals affect drug therapy responses. In general, dosages are based on a weight of approximately 150 lb, which is calculated to be the “average” weight of men and women.
For many drugs higher than average dosages may be necessary to produce the desired effect in an overweight individual. An underweight individual, on the other hand, may require a smaller dosage to produce the desired effect.
Ethnicity changes the expected effects of drugs in individuals. For example, many people of Asian descent have a greater-than-usual response to the anticoagulant drug warfarin, which greatly increases their risk for excessive bleeding, hemorrhage, and stroke. As a result, the starting dosage of warfarin is lower for Asians and increased at a slower rate until the individual patient’s response is known.
The age of the patient may influence the effects of a drug. For instance, infants and children require smaller doses of a drug than adults do. This is because they have smaller fat and total water content, immature enzyme systems, reduced kidney function, immature livers, and variation in circulating blood proteins which affects their ability to metabolize drugs.
Elderly patients may also require smaller dosages because of decline in liver size, blood flow, and enzyme production as well as the presence of several disease processes, and the necessity for many medications.
The gender of an individual may influence the action of some drugs. Women may require a smaller dose of some drugs than men. This is because many women are smaller than men and have a body fat-and-water ratio different from that of men.
There is great variation in the activity of cytochrome P-450 enzyme system from one person to the next as a result of genetic differences. Some persons may be rapid metabolizer of drugs and require higher-than-expected dosages to achieve the same desired effect.
Others may be slow metabolizer and have more problems with side effects, adverse reactions, and toxic effects even from what are considered “normal” drug dosages.
The presence of certain diseases may influence the action of some drugs. Sometimes disease is an indication for not prescribing a drug or for reducing the dose and frequency of administration of certain drugs.
In liver (hepatic) disease, for example, the ability to metabolize or detoxify a specific type of drug may be impaired. If the average or normal dose of the drug is given, the liver may be unable to metabolize the drug at a normal rate. Consequently, the drug may be excreted from the body at a much slower rate than normal.
Patients with kidney disease may exhibit drug toxicity and a longer duration of drug action. The dosage of drugs and frequency of administration may be reduced to prevent the accumulation of toxic levels in the blood or further injury to the kidney.
Use of other prescribed, over-the-counter, or illicit drugs, as well as alcohol intake often increase the activity of metabolic enzyme systems. This change increases the rate at which some drugs are deactivated and eliminated, often requiring that the doses be increased and/or given more frequently to be effective.
For example, opioid drugs for pain are metabolized and eliminated much faster in the person who drinks alcohol on a daily basis. The dose then may need to be higher for the person to obtain pain relief.
The article “Why do the effects of drugs vary between different people?” provides answers to the following questions Why do drugs not have the same effect on all patients? Why do medications affect people differently? Why do drugs not have the same effect on all patients what part does age play? Why do drugs affect me more than others? Why do some drugs affect individuals in different ways? How does weight influence drug effectiveness? Why do people respond differently to drugs?